X-ray imaging apparatus, method of controlling the same, and computer program

ABSTRACT

An X-ray imaging apparatus includes an X-ray source including a plurality of X-ray focuses, an X-ray detector which detects X-rays emitted from the X-ray focuses and transmitted through an object, and a control unit which controls the X-ray source and the X-ray detector. The X-ray imaging apparatus selects a pair of X-ray focuses, of X-ray focuses of the plurality of X-ray focuses which project images on the X-ray detector through a region of interest which is an imaging region of the object, from which emitted X-rays define an intersecting angle coinciding with a predetermined angle in the region of interest, and decides an X-ray focus to be used for imaging from X-ray focuses between the selected pair of X-ray focuses. An X-ray image is captured by emitting X-rays from the decided X-ray focus and causing the X-ray detector to detect the X-rays.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a CONTINUATION of co-pending U.S. patent applicationSer. No. 13/082,613 filed 8 Apr. 2011, which is a CONTINUATION of priorU.S. patent application Ser. No. 12/881,972 filed 14 Sep. 2010, now U.S.Pat. No. 794,088 issued 10 May 2011, which is a CONTINUATION ofInternational application No. PCT/JP2010/001988 filed 19 Mar. 2010,which claims priority from Japanese Patent Application No. 2009-091480filed 3 Apr. 2009. The disclosures of the above-referenced applicationsand patent are hereby incorporated by reference herein in theirentirety.

TECHNICAL FIELD

The present invention relates to an X-ray imaging apparatus, a method ofcontrolling the same, and a computer program for the above.

BACKGROUND ART

A tomography apparatus controls the motion range of an X-ray apparatusor two-dimensional detector in accordance with an imaging region.Japanese Patent Application Laid-Open No. 2004-041702 (PLT1) andInternational Patent Application Publication WO/2007/100105 (PLT2) arerelated to such a tomography apparatus. PLT1 discloses a radiographicapparatus that radiographs an object with radiation from a movableradiation source. The radiographic apparatus detects a radiographicimage of an object supported by a support portion by using a detectionunit. At this time, based on the information of the position of theradiation source and an imaging target region of an object, a controlunit controls the movement of at least one of the detection unit and thesupport portion so as to detect a projection image of the imaging targetregion.

A tomography apparatus designed to radiograph an object with radiationfrom a radiation source controls each of the electron sources of theradiation source, which are distributed two-dimensionally, to generatemulti X-ray beams, and detects a radiographic image of the object byusing a detector (see PLT2). Using such multi X-ray beams can performtomography without moving both the radiation source and the detector.

CITATION LIST Patent Literature

-   PLT1: Japanese Patent Application Laid-Open No. 2004-041702-   PLT2: International Patent Application Publication WO/2007/100105

In tomography, however, it is impossible to maintain the resolution inthe Z-axis (thickness) direction unless imaging is performed at apredetermined tomographic angle or more. There has been provided notechnique capable of performing tomography in a limited space such as anoperating room while maintaining a predetermined tomographic angle ormore. There has been proposed no effective technique of selecting aplurality of X-ray sources when performing tomography at a predeterminedtomographic angle or more.

The present invention provides a technique capable of performingtomography while maintaining a predetermined tomographic angle or morewithout simultaneously moving an X-ray apparatus and a two-dimensionaldetector at the time of imaging.

SUMMARY OF INVENTION

In order to solve the above problem, according to the present invention,there is provided an X-ray imaging apparatus including an X-ray sourceincluding a plurality of X-ray focuses, an X-ray detector which detectsX-rays emitted from the X-ray focuses and transmitted through an object,and control means for controlling the X-ray source and the X-raydetector, the apparatus comprising,

selection means for selecting a pair of X-ray focuses, of X-ray focusesof the plurality of X-ray focuses which are configured to project imageson the X-ray detector through a region of interest which is an imagingregion of the object, from which emitted X-rays define an intersectingangle coinciding with a predetermined angle in the region of interest,and

decision means for deciding an X-ray focus to be used for imaging fromX-ray focuses between the pair of X-ray focuses selected by theselection means,

wherein the control means captures an X-ray image by causing emission ofX-rays from the X-ray focus decided by the decision means and causingthe X-ray detector to detect the X-rays.

The present invention can provide a technique capable of performingtomography while maintaining a predetermined tomographic angle or morewithout simultaneously moving an X-ray apparatus and a two-dimensionaldetector at the time of imaging.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a perspective view showing an example of the arrangement of anX-ray imaging system according to an embodiment of the presentinvention;

FIG. 2 is a view for explaining the structure of a multi X-ray source 26according to the embodiment of the present invention;

FIG. 3 is a view for explaining a method of deciding a region ofinterest according to the embodiment of the present invention;

FIG. 4 is a flowchart for X-ray focus selection processing according tothe embodiment of the present invention;

FIG. 5 is a view for explaining a method of selecting X-ray focusesaccording to the embodiment of the present invention;

FIG. 6 is a side view of a stop unit according to the embodiment of thepresent invention; and

FIG. 7 is a plan view of the stop unit according to the embodiment ofthe present invention.

DESCRIPTION OF EMBODIMENTS

A preferred embodiment of the present invention will be described indetail with reference to the accompanying drawings. FIG. 1 shows a casein which an X-ray imaging system 10 according to the embodiment of thepresent invention is capturing an X-ray tomogram of a human body. Atwo-dimensional X-ray detector 28 and a multi X-ray source 26 are fixedto a C-ram 25. A stop unit 27 is fixed on the emission side of the multiX-ray source 26. The multi X-ray source 26 is placed on the backside ofa patient. The multi X-ray source 26 includes N×M X-ray focuses whichare arranged two-dimensionally. The X-rays emitted from a transmissiontarget of the multi X-ray source 26 are transmitted through a human body34 as an object and reach the two-dimensional X-ray detector 28. Thetwo-dimensional X-ray detector 28 outputs the intensity distribution ofthe incident X-rays, calculates an X-ray tomogram, and displays it on adisplay unit 31. A control panel 30 is connected to a control unit 29.The control unit 29 decides a region of interest (to be described later)and an imaging mode for tomography (a maximum tomographic angle mode orfixed tomographic angle mode) in accordance with the operation by adoctor via the control panel 30. In addition, a tomogram is calculatedbased on the image read from the two-dimensional X-ray detector 28 andis displayed on the display unit 31.

The structure of the multi X-ray source 26 will be described next withreference to FIG. 2. The multi X-ray source 26 includes an element array16. Each of multi electron emission elements 15 arranged on the elementarray 16 emits electrons. Emitted electrons are shaped by a lenselectrode 19 and accelerated by an accelerating electric field tocollide with a transmission target 13. An in-vacuum X-ray shield 23limits the direction of the X-rays generated by the transmission target13. It is possible to further limit the direction of the X-rays by usingan in-atmosphere X-ray shield 41. Note however that this embodiment usesthe stop unit 27 in place of the in-atmosphere X-ray shield 41. Anarrangement using the stop unit 27 will be described later withreference to FIGS. 6 and 7.

A method of deciding a region of interest in this embodiment will bedescribed next. A region of interest is a region which corresponds to animaging region of the object 34 from which an X-ray tomogram is to becaptured, and through which X-rays emerging from a plurality of X-rayfocuses of the multi X-ray source 26 are commonly transmitted. It ispossible to decide a region of interest by the following three types ofmethods. The first method is a method of making the operator set theapparatus by eye measure to match with specified values set in advanceby the control unit 29 including a computer. In this case, the specifiedvalues include the distance from the two-dimensional X-ray detector 28to a region of interest, the distance from the multi X-ray source 26 tothe region of interest, and the size of the region of interest. Thesecond method is a method of making the operator input visually measuredvalues to the control unit 29 after setting the apparatus for a patient.The third method is a decision method using X-ray images. A merit of thethird method is that it involves few errors.

A method of deciding a region of interest in the third method will bedescribed next with reference to FIG. 3. The items which should bedecided regarding a region of interest 33 include the distance from thetwo-dimensional X-ray detector 28 to the region of interest 33, thedistance from the multi X-ray source 26 to the region of interest 33,and the size of the region of interest 33. The size of the region ofinterest 33 may be expressed in terms of a sphere or rectangularparallelepiped. In this case, for the sake of convenience, the size isexpressed in terms of a rectangular parallelepiped, and the distancesfrom the two-dimensional X-ray detector 28 and the multi X-ray source 26to the region of interest 33 are those to the center of a cube as an endpoint. This apparatus captures X-ray images from an exposure focus A andan exposure focus B before tomography. The operator can designate pointsof interest (analytical portions of interest indicated by the bullets)301 and 302 on the two X-ray images obtained from the X-rays from therespective exposure focuses. Referring to FIG. 3, an intersecting angleθ between a line segment connecting the exposure focus A and the pointof interest 301 and a line segment connecting the exposure focus B andthe point of interest 302 will be referred to as a tomographic angle 35.

An X-ray image is output to the display unit 31. Since the X-ray imagecorresponds to the detection surface of the two-dimensional X-raydetector 28, connecting the exposure focuses A and B to the points ofinterest 301 and 302 with straight lines can obtain the center of theregion of interest 33. Assume that the distance (FDD: Focus DetectorDistance) from the center of the multi X-ray source 26 to thetwo-dimensional X-ray detector 28 is known. In addition, the distancesbetween the exposure focus A and the exposure focus B and the distancebetween the point of interest 301 and the point of interest 302 are alsoknown. It is possible to obtain the distance (CDD: Center DetectorDistance) from the two-dimensional X-ray detector 28 to the region ofinterest 33 and the distance (FCD: Focus Center Distance) from the multiX-ray source 26 to the region of interest 33 based on these ratios. Theoperator can designate the size of the region of interest 33 with amouse or the like. In this embodiment, the region of interest 33 isassumed to be a cube. Therefore, the operator can designate the size ofthe region of interest 33 (one side of the cube: P) even by designationon an X-ray image plane.

In addition, it is possible to automatically decide the region ofinterest 33 by image processing. During a surgical operation, it ispossible to place a radiopaque object (bead) in the actual region ofinterest 33. It is therefore only required to search for a radiopaqueobject by image processing. If it is impossible to place a radiopaqueobject, a high-contrast lesion or medical treatment component can be setas a target for a point of interest. According to the above description,two X-ray images are used. However, it is possible to use three or moreX-ray images to improve the accuracy.

A method of selecting X-ray focuses to be used for imaging from aplurality of X-ray focuses will be described next with reference to theflowchart of FIG. 4 and the schematic view of FIG. 5. First of all, instep S401, the operator sets the X-ray imaging system 10 shown in FIG. 1for the object (patient) 34. In step S402, the region of interest 33 isdecided by performing X-ray irradiation in the above manner. Notehowever that methods of deciding the region of interest 33 include amethod without the above X-ray irradiation. In step S403, this systemsets an imaging mode based on input operation by the operator. Thisembodiment includes two imaging modes. One is a mode of imaging uponsetting the tomographic angle 35 to the maximum in tomography. The otheris a mode of imaging upon setting the tomographic angle 35 to apredetermined angle as a fixed value (preset value). The tomographicangle 35 influences the resolution of a tomogram in the Z direction (thethickness direction of the region of interest 33 or the direction fromthe multi X-ray source 26 to the detector 28).

In step S404, the system determines the set mode. If the mode of settingthe tomographic angle 35 to the maximum is set, the process shifts tostep S405. If the mode of setting the tomographic angle 35 to apredetermined angle as a fixed value, the process shifts to step S408.

The mode of imaging upon setting the tomographic angle 35 to the maximumin steps S405 to S408 will be described first. In the mode of imagingupon setting the tomographic angle 35 to the maximum, the systemselects, in step S405, outermost X-ray focuses of the multi X-ray source26 which maximize the tomographic angle 35. In step S406, when X-raysemerge from the selected X-ray focuses so as to intersect each other atthe center of the region of interest 33, the system determines whethertransmitted X-rays fall outside the two-dimensional X-ray detector 28(vignetting occurs). In this determination, the system uses the ratiobetween a distance CDD from the two-dimensional X-ray detector 28 to theregion of interest 33 and a distance FCD from the multi X-ray source 26to the region of interest 33. That is, the system obtains an X-rayirradiation region of the two-dimensional X-ray detector 28 by using thelength between X-ray focuses and the ratio when the tomographic angle 35is set to the maximum. If the irradiation region is larger than thedetection surface of the two-dimensional X-ray detector 28, the systemcan determine that vignetting will occur. If no vignetting will occur(NO in step S406), the system decides to select specific X-ray focusesinside the outermost X-ray focuses based on the selected pitch in stepS408. 5 a in FIG. 5 indicates a case in which transmitted X-rays do notfall outside the detection surface of the two-dimensional X-ray detector28. If vignetting will occur (YES in step S406), the system selectsX-ray focuses, of the X-ray focuses of the multi X-ray source 26, whichare located more inwardly again in step S407. The process then returnsto step S406 to continue the processing. 5 b in FIG. 5 indicates a casein which when transmitted X-rays fall outside the two-dimensional X-raydetector 28, the outermost X-ray focuses are limited to prevent thetransmitted X-rays from slipping from detection.

In this embodiment, the system repeats the processing from step S405 tostep S407, and selects a pair of X-ray focuses, of the X-ray focuseswhich allow to project images on the two-dimensional X-ray detector 28through a region of interest without causing vignetting, which arelocated outermost of the multi X-ray source. This makes it possible tospecify a pair of X-ray focuses from which X-rays transmitted throughthe region of interest 33 decided in step S402 can be emitted and whichcan maximize the tomographic angle 35.

The processing in step S408 will be described below. In tomography, theimage resolution in the Z-axis direction depends on the tomographicangle 35, and SN(Signal to Noise) depends on the number of frames to becaptured. Note however that increasing the number of frames willincrease the amount of exposure to the patient. If a number N (N is aninteger equal to or more than 2) of frames is set, a selection anglepitch ρ can be expressed by θ/(N−1) based on a tomographic angle θ35 andthe number N of frames. It is possible to approximate a selection pitchp of X-ray focuses (intervals at which X-ray focuses are selected) onthe multi X-ray source 26 by expression (1) given below using thedistance FCD from the multi X-ray source 26 to the region of interest33.

p≈FCD*tan(θ/(N−1))  (1)

However, since actual arrangement intervals w of the X-ray sources onthe multi X-ray source 26 are physically fixed, p/w is not necessarilyan integer. In this embodiment, it is possible to express an X-ray focusto be selected as X(trunc(n*(p/w))+a) by letting n be a natural number(n=1, 2, . . . ) and dropping the fractional portion of n*(p/w). Notethat a represents an offset value for specifying the first X-ray focusto be selected, and takes an integral value. Note that if the selectionangle pitch ρ is a fixed value, a selection pitch p1 on the multi X-raysource 26 can be approximated by expression (2) regardless of thetomographic angle θ35.

p1≈FCD*tan(ρ)  (2)

An X-ray focus to be selected at this time is expressed byX(trunc(n*(p1/w))+a). It is important in this case that even if eitherthe number N of frames in tomography or the selection angle pitch ρ isfixed, the X-ray source pitch of the multi X-ray source 26 is decideddepending on FCD. If X-ray focuses to be used at the time of X-rayimaging are decided in the above manner, this system captures X-rayimages in step S409 by sequentially emitting X-rays from the decidedX-ray focuses and making the control unit 29 receive transmitted X-rayimages from the two-dimensional X-ray detector 28. In this case, themulti X-ray source 26 can simultaneously emit X-rays from a plurality ofX-ray focuses. However, in order to avoid a reduction in contrast due toscattered radiation, the multi X-ray source 26 preferably emits X-rayswhile sequentially switching X-ray focuses. It is possible to set theswitching timing to about 30 msec. When using, for example, 30 X-rayfocuses, the system can perform tomography in one sec. When the systemis to perform high-speed imaging, the stop unit 27 prepares in advancestop apertures corresponding to a plurality of X-ray focuses selectedbefore the start of imaging.

The mode of imaging upon setting the tomographic angle 35 to the fixedvalue (preset value) in steps S410 to S414 will be described next. Firstof all, in step S410, the system calculates X-ray focuses that make thetomographic angle 35 coincide with a predetermined angle. The systemperforms this calculation by using the position and size of the regionof interest 33, the distance FCD from the center of the multi X-raysource 26 to the region of interest 33, the distance CDD from thetwo-dimensional X-ray detector 28 to the region of interest 33, and thesizes of the multi X-ray source 26 and two-dimensional X-ray detector28. Assume a geometric system in which a vertical line passing throughthe center of the two-dimensional X-ray detector 28 passes through thecenter of the multi X-ray source 26. Assume also that selected X-rayfocuses are arranged to be point-symmetric about the centers on themulti X-ray source 26 and two-dimensional X-ray detector 28.

In step S411, the system determines whether the X-ray focuses decided instep S410 can be selected on the multi X-ray source 26. If they can beselected, the process shifts to step S413. If there are no X-ray focuseswhich can be selected, the process shifts to step S412. For example, asindicated by 5 c in FIG. 5, when a region of interest is located nearthe two-dimensional X-ray detector 28, the positions decided in stepS410 may fall outside the multi X-ray source 26. In such a case, thedisplay unit 31 displays an error indicating that the X-ray focusescannot be set (cannot be selected) in step S412. The display unit 31also displays the current values of the distances FCD and CDD and thedesired values of the respective distances for allowing the X-rayfocuses to be selected. This allows the operator to adjust thepositional relationship between the multi X-ray source 26 and thetwo-dimensional X-ray detector 28 by referring to the desired valuesdisplayed on the display unit 31. The process therefore returns to stepS410 to perform processing in the positional relationship between themulti X-ray source 26 and the two-dimensional X-ray detector 28 afteradjustment corresponding to the error display in step S412. Note that itis possible to perform position adjustment automatically instead ofmanually.

A case in which the system determines in step S411 that the X-rayfocuses can be selected will be described next. In this case, theprocess shifts to step S413 to determine whether transmitted X-rays fromthe selected X-ray focuses fall outside the two-dimensional X-raydetector 28 (vignetting occurs). The determination method in this caseis the same as that in step S406. As indicated by 5 d in FIG. 5, forexample, if no vignetting occurs (NO in step S413), the process shiftsto step S408. In step S408, the system calculates a selection pitchaccording to expressions (1) and (2) as described above, and selectsX-ray focuses to be used for tomography. In contrast, as indicated by 5e in FIG. 5, if vignetting occurs (YES in step S413), the process shiftsto step S414. In step S414, the display unit 31 displays the currentvalues of the distances FCD and CDD and desired values for theprevention of vignetting at the same time when performing error display.This allows the operator to adjust the positional relationship betweenthe multi X-ray source 26 and the two-dimensional X-ray detector 28 byreferring to the desired values displayed on the display unit 31 as inthe case of step S412. The process returns to step S410 to performprocessing in the positional relationship between the multi X-ray source26 and the two-dimensional X-ray detector 28 after adjustmentcorresponding to the error display in step S414. Note that it ispossible to perform position adjustment automatically instead ofmanually.

Although the above case has been described in the one-dimensional methodwith reference to FIG. 5 for the sake of convenience, it is possible toextend the method executed in a one-dimensional manner to atwo-dimensional method. The above description is the case in which thesystem performs error display in step S412 or S414 to allow the operatorto manually adjust the positional relationship between the multi X-raysource 26 and the two-dimensional X-ray detector 28 or to automaticallyadjust the positional relationship. However, the embodiment of thepresent invention is not limited to the adjustment of the positionalrelationship. For example, it is possible to change the fixed value ofthe tomographic angle 35. More specifically, the value of thetomographic angle 35 may be decreased by a predetermined value. In thiscase, if the system determines in step S411 that the X-ray focusescannot be selected or determines in step S413 that vignetting occurs,the system sequentially decreases the value of the tomographic angle 35a predetermined value at a time to narrow down the value to a value thatallows the X-ray focuses to be selected and prevents the occurrence ofvignetting.

The function of the stop unit 27 will be described next with referenceto FIG. 6. For the sake of descriptive simplicity, the description madewith reference to FIG. 6 is limited to one-dimensional directions.However, since operations in the respective dimensions are independentof each other, it is possible to easily extend one-dimensional operationto two-dimensional operation. 6 a in FIG. 6 indicates a case in whichthe system controls a stop plate 32 to make the respective X-ray sourcesconstituting the multi X-ray source 26 have irradiation regions in thesame place on the two-dimensional X-ray detector 28. The stop plate 32is a member to shield X-rays, and is made of tungsten, lead, copper,iron, or an alloy of them. As shown in FIG. 7, the stop plate 32 can beconstituted by four types of stop plates 32A, 32B, 32C, and 32D. Thestop unit 27 includes the set of stop plates 32 and a driving unit (notshown) to drive them. Referring to FIG. 6, vacuum partitions 14 areprovided between the stop unit 27, the transmission target 13, and thein-vacuum X-ray shield 23.

6 b in FIG. 6 indicates a case in which the system performs tomographyof the region of interest 33 by using the stop unit 27. The control unit29 controls the stop plates to make the irradiation regions formed by atransmission target ti and another transmission target tj include theregion of interest 33, thereby forming X-ray transmission windows. It ispossible to perform X-ray transmission window formation control usingthe stop plate 32 for only portions (stop apertures 42) corresponding tothe selected transmission targets (X-ray focuses). It is howeverpossible to change stop apertures which are not selected for the sake ofcontrol. FIG. 7 shows an example of control of the stop plate 32. 7 aand 7 b in FIG. 7 correspond to 6 a and 6 b in FIG. 6 and indicate acase in which tomography is performed upon reduction of the irradiationfield. It is preferable to complete control of the stop plate 32 toobtain an appropriate size of stop apertures 42 before the start oftomography. This makes it possible to perform high-speed tomography andavoid image deterioration due to the movement of a patient or organ.

As described above, according to the embodiment of the presentinvention, it is possible to perform tomography at high speed by usingthe multi X-ray source without simultaneously moving (displacing) theX-ray sources or the two-dimensional X-ray detector.

The present invention is also implemented by executing the followingprocessing. This processing is the processing of supplying software(programs) for implementing the functions of the above embodiment to asystem or apparatus via a network or various kinds of storage media, andcausing the computer (or the CPU, MPU, or the like) of the system orapparatus to read out and execute the programs.

The present invention is not limited to the above embodiment and variouschanges and modifications can be made within the spirit and scope of thepresent invention. Therefore, to apprise the public of the scope of thepresent invention, the following claims are made.

1. (canceled)
 2. An X-ray imaging apparatus for capturing an X-raytomogram of an object, comprising: a selection unit configured to selecttwo X-ray focuses out of the plurality of X-ray focuses which projectimages on an X-ray detector through a region of interest which is animaging region of the object, the selected two X-ray focuses having anintersecting angle of a predetermined value in the region of interest;and a control unit configured to control the X-ray focuses existingbetween the selected two X-ray focuses to emit X-rays and to control theX-ray detector to detect the emitted X-rays, for capturing an X-rayimage.
 3. The X-ray imaging apparatus according to claim 2, wherein,when the selected two X-ray focuses are not included in the plurality ofX-ray focuses, the selection unit selects the two X-ray focuses againupon decreasing the predetermined value of the intersecting angle. 4.The X-ray imaging apparatus according to claim 2, wherein, when theintersecting angle of the selected two X-ray focuses does not match thepredetermined value, the selection unit selects the two X-ray focusesagain upon decreasing the predetermined value of the intersecting angle.5. The X-ray imaging apparatus according to claim 2, further comprisinga display unit configured to perform error display when the two X-rayfocuses are not included in the plurality of X-ray focuses, wherein,when a positional relationship between an X-ray source and the X-raydetector is changed in accordance with the error display, the selectionunit selects the two X-ray focuses again.
 6. The X-ray imaging apparatusaccording to claim 5, wherein contents displayed by the error displayinclude a current value and a desired value of a distance between anX-ray source and the region of interest, and a current value and adesired value of a distance between the X-ray detector and the region ofinterest.
 7. An X-ray imaging apparatus, for capturing an X-ray tomogramof an object, that includes an X-ray source having a plurality of X-rayfocuses of a transmission target, the apparatus comprising: a selectionunit configured to select a pair of X-ray focuses out of the pluralityof X-ray focuses which project images on an X-ray detector through aregion of interest which is an imaging region of the object, theselected pair of X-ray focuses being located outermost of the pluralityof X-ray focuses; and a control unit configured to control the X-rayfocuses existing between the selected pair of X-ray focuses to emitX-rays and to control the X-ray detector to detect the emitted X-rays,for capturing an X-ray image.
 8. An X-ray imaging apparatus, forcapturing an X-ray tomogram of an object, that includes an X-raydetector which detects X-rays emitted from X-ray focuses and transmittedthrough the object, the apparatus comprising: a selection unitconfigured to select a pair of X-ray focuses out of the plurality ofX-ray focuses which project images on the X-ray detector through aregion of interest which is an imaging region of the object, theselected pair of X-ray focuses being located outermost of the pluralityof X-ray focuses; and a control unit configured to control the X-rayfocuses existing between the selected pair of X-ray focuses to emitX-rays and to control the X-ray detector to detect the emitted X-rays,for capturing an X-ray image.
 9. A method of controlling an X-rayimaging apparatus for capturing an X-ray tomogram of an object, themethod comprising steps of: selecting two X-ray focuses out of theplurality of X-ray focuses which project images on an X-ray detectorthrough a region of interest which is an imaging region of the object,the selected two X-ray focuses having an intersecting angle of apredetermined value in the region of interest; and controlling the X-rayfocuses existing between the selected two X-ray focuses to emit X-raysand to control the X-ray detector to detect the emitted X-rays, forcapturing an X-ray image.
 10. A non-transitory computer-readable storagemedium storing thereon a computer-executable program for causing acomputer to control an X-ray imaging apparatus according to the methodof claim 9.